First-principle Study Of Hydrogen Permeation Behavior For α-Al₂O₃and Adsorption Behavior On Graphite For Nuclides

In the thesis, using the first-principles software-VASP, hydrogen adsorption,diffusion and trapping on the reconstructive surface and in the bulk of α-Al2O3crystalhave been studied. It turns out that H atoms will be chemically absorbed with thebinding energy of2.29eV on the Al terminated reconstructive surface and can releaseby overcoming the energy barrier of1.01eV in the way of forming H2molecules whoseadsorption belongs to physisorption. An O (Al) vacancy can trap hydrogen up to4(7)atoms in which1(2) H2molecule(s) is (are) formed. The H interstitial atom lies in thedodecahedral interstice with the formation energy of0.75eV, and it can accommodate ahydrogen molecule at most. Diffusing from the constructive surface to sub-surfaceshould have to overcome an energy barrier of1.73eV and the activation energy fromone dodecahedron to another is1.12eV in the bulk of α-Al2O3. The possiblemechanisms against permeation of hydrogen in α-Al2O3crystals are discussed and theseproperties would make α-Al2O3crystals as one of the excellent candidates.In the other hand, the adsorption and desorption of four kinds of main radioactiveproductions (cesium, iodine, strontium and silver) on graphite surface in hightemperature gas cooled reactors (HTR) have been studied. Adsorptive geometry(including adsorptive site, height and so on), energy and electron structure on theperfect and defective graphite surfaces have been calculated. It turns out that theadsorption of Cs, I and Sr atoms belongs to chemisorption while the adsorption of Ag isa pure physisorption. Electronic structure analysis reveals that Cs donates its partial6sand5p states to graphite substrate, Sr donates its partial electrons of4p and5s states tographite substrate while the5p state of I accepts electron from carbon atoms. As for Agnuclide, there is no any electron transfer between Ag and graphite. When introducing avacancy in graphite surface, nuclide adatoms will be trapped by the vacancy and formchemical bonds with three nearest neighbor carbon atoms, leading to significantincrease of the adsorption energy. In addition, a model of grand canonical ensemble isemployed to deduce the adsorption rate as a function of adsorption energy, temperatureand partial pressure of nuclides produced. The transition temperate from adsorption todesorption of nuclides on graphite surface is defined as the inflexion point of theadsorption rate and its variation with nuclide density is obtained. When nuclide density n=1×1016m-3in the primary circuit, the critical temperature is about416K,162K,161K and13K for Cs, I, Sr and Ag respectively.